The present invention provides an improved rotary position sensor for monitoring the angular position of a rotating shaft.
In mechanical systems, motion is often initiated and controlled by rotating a member such as a shaft. The angular motion of the shaft is then translated into some other motion, such linear displacement, rotation of a pump or fan, or the angular rotation of some other intermediate part at a different angular velocity or spatial orientation. Numerous mechanical means such as gears, cams, pulleys, and belts are commonly employed in translating the angular motion of an input shaft to drive an output device. One simple example is that of an automotive rack and pinion steering system. In a simplified steering system, the input shaft comprises the rotating steering column driven by the manual input of a human operator turning the steering wheel. As the steering wheel is turned, the steering column rotates, driving a pinion attached to the end of the shaft. The pinion meshes with a rack which is pulled from side to side in a linear motion directed perpendicular to the steering column. Tie rods attached to the rack connect to the front wheel hubs, and as the rack moves back and forth the tie rods push and pull on the front wheel hubs, turning the front wheels in response to the rotation of the steering wheel. While most modern power steering systems are much more complex than the skeletal system just described, the above example illustrates a simple mechanical system wherein the angular rotation of an input shaft (the steering column) is translated into a completely different motion (rotation of the front wheels of the car). What should also be clear from the above example, is that the amount of angular displacement of the front wheels is entirely dependent on the angular displacement of the steering column.
Often it is desirable to monitor the position of various mechanical parts within a mechanical system. However, in many cases due to space restrictions or other physical characteristics, it is inconvenient or impossible to directly monitor the position of a particular part. In such cases it is often easier to monitor the position of the part indirectly. For example, in the steering mechanism described above, measuring the angular position of the front wheels directly would be a difficult and expensive proposition, but because of the rigid mechanical link between the steering column, the pinion, the rack, the tie rods and the wheels, the output position of the wheels can be accurately determined by monitoring the angular position of the steering column. Thus, by supplying a rotary position sensor on the steering column of an automobile it is possible to generate an electrical signal which indicates the angular position of the front wheels.
This relationship between the angular position of a rotating input shaft and the position of an output or intermediate mechanical member is ubiquitous throughout the mechanical arts. In some applications, such as servo motors, a position sensor is mounted directly to the output shaft of a motor, and the output position and/or speed of the machine can be readily determined by monitoring the rotation of the motor. In any mechanical system wherein the output position of a mechanical part is to be determined by the position of a rotating input shaft, a key element is the rotary position sensor. The rotary position sensor must accurately and reliably determine the angular position of the input shaft before that information can be extrapolated into the position of the output member. In addition to accuracy and reliability issues, each specific application will provide its own demands and limitations on the design of the rotary position sensor. For example, in the steering system described above, the steering wheel may be rotated several times in turning the front wheels from their maximum left turn position to their maximum right turn position. Thus, a rotary position sensor for this system must function over a number of turns of the input shaft. The sensor must be able to determine not only the angular position of the steering wheel, but also the position of the wheel within its full turning range. In other systems it may only be necessary to sense rotation over a single turn. In still other applications, physical constraints may make it difficult to couple electrical signals to the rotating portion of the sensor. And finally, the cost of various position sensors may be an overriding factor in determining the best sensor for a particular application.
Some rotary position sensors currently in use include rotary potentiometers, inductive position resolvers, and optical encoders. Each of these devices have their own characteristic advantages and disadvantages, which make them more suitable for some applications rather than others. Rotary potentiometers, for example supply a voltage signal proportional to the position of a wiper contact which rides along a resistive element. Initially, such rotary potentiometers are quite accurate and provide excellent position indication over a single turn of the input shaft. However, over time, the sliding motion of the wiper contact over the resistive element can lead to wear which alters the resistance ratio between the resistive element and the wiper contact, leading to inaccuracy in the output position signal. Rotary potentiometers are also subject to contamination of the contact elements which can adversely effect the accuracy of the device. For these reasons, rotary potentiometers are not well suited for those applications where extended long term reliability is required or where harsh environmental conditions are likely to adversely effect the sensor. Thus, a rotary potentiometer would be particularly unsuited for application in the steering mechanism described above. Because of the near constant back and forth motion of the steering wheel around the point where the front wheels are approximately straight, and the only occasional wider deviations during the execution of sharper turns, the resistive element within the rotary potentiometer will tend to wear unevenly so that the output position signal will no longer be linear over the entire range of the wiper contact.
Inductive position resolvers, on the other hand, have advantages over rotary potentiometers in that they are non-contact devices. Resolvers operate on inductive principles, having mutually coupled coils mounted to both a rotor and a stator. As the rotor coil rotates relative to the stator coil, the mutual inductance between the two coils changes such that a voltage signal impressed on the stator coil will be coupled to the rotor coil in varying strength depending on the angular relationship between the coils. While resolvers have obvious advantages over rotary potentiometers, a drawback is that they require signal connections to the rotating member. Therefore, slip rings or some other mechanism for connecting electrical signals to the rotating member are required. Also, resolvers are generally more expensive than rotary potentiometers and more sensitive to vibration and shock.
Finally, optical encoders are often used as rotary position sensors, but they also offer significant drawbacks for certain applications. As with resolvers, optical encoders tend to be expensive, thus making them inappropriate for those applications where low cost is a critical design factor. Furthermore, encoders are digital devices, emitting light pulses for each fraction of a rotation of the input shaft. The resolution of an encoder is determined by physical limitations in the number of pulses which can be generated per revolution of the input shaft. Thus, optical encoders are inappropriate for applications wherein a continuous analog signal is required.
In many applications a low cost durable analog sensor is required. In most such cases a non-contact type sensor is preferred wherein the rotating member within the sensor has no physical contact with the stator in order to prevent wear between the rotating parts. In some cases it may be necessary to provide a non-contact rotary sensor wherein there are no electrical connections required for coupling signal to the rotating member. In still other cases it may at be necessary to provide a rotary position sensor wherein the sensor provides a continuous analog signal over a multi-turn range of the input shaft.
Returning to the example of the steering system as described above, it is desirable to provide a rotary position sensor for determining the angular position of the steering column. Since the steering wheel will rotate several times over the full turning range of the front wheels, a position sensor must be capable of determining position over a number turns of the steering wheel. Also, due to the difficulty in coupling electrical signals to a rotatable steering wheel, a position sensor which requires no electrical connections, or at least very few electrical connections to the rotating member is desirable. In a steering mechanism, an additional improvement would be to provide a rotary position sensor which can be housed within an airbag deployment connector known as a clockspring.